1. Field of the Invention
The present invention relates generally to thermal barrier coatings, and more specifically to a TBC on a turbine airfoil.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine, such as an aero engine used to power an aircraft and an industrial gas turbine engine used to produce electric power, includes a turbine section with a plurality of stages of stator vanes interspersed with an equal number of stages of rotor blades to extract energy from a hot gas flow passing through. It is well known that the efficiency of the engine can be increased by providing for a higher gas flow into the turbine. However, the turbine, especially the first stage stator vanes and rotor blades, limit how high the gas flow temperature can be. If the temperature is too high, these airfoils can be severally damaged.
Thus, airfoil designers attempt to provide the airfoils with complex internal air cooling passages to allow for these airfoils to be exposed to higher temperatures, or apply thermal barrier coatings (TBC) to the outside surface that is exposed to the high temperature gas flow in order to protect the airfoil. A combination of air cooling and TBC is used to allow for higher temperatures. In the prior art, a thin TBC is used in the turbine airfoil cooling design to provide insulation for the airfoil for the reduction of heat load from the hot gas flow to the airfoil which reduces the airfoil metal temperature and therefore reduces the cooling flow consumption and improves the turbine efficiency. As the turbine inlet temperature increases, the cooling flow demand for cooling the airfoil will increase and therefore reduce the turbine efficiency. One method of reducing the cooling air consumption while increasing the turbine inlet temperature for higher turbine efficiency is by using thicker TBC on the cooled airfoil surface. The cooling design becomes more reliant on the coating's endurance, and the TBC becomes the “prime reliance” in the cooling design. The problem with this approach is that a thicker TBC increases the chance of spallation.
A TBC applied to the airfoil surface is typically about 0.4 mm in thickness and has no reinforcement. A thicker layer of TBC would provide better insulation to the airfoil surface, but using a thicker layer than this would produce spallation in the TBC. Spallation is when ships of the TBC break away and leave holes in the insulation. U.S. Pat. No. 6,428,280 B1 issued to Austin et al on Aug. 6, 2002 entitled STRCUTURE WITH CERAMIC FOAM THERMAL BARRIER COATING, AND ITS PREPARATION shows this typical TBC on an airfoil surface.
U.S. Pat. No. 6,551,061 B2 issued to Darolia et al on Apr. 22, 2003 entitled PROCESS FOR FORMING MICRO COOLING CHANNELS INSIDE A THERMAL BARRIER COATING SYSTEM WITHOUT MASKING MATERIAL and U.S. Pat. No. 6,617,003 B1 issued to Lee et al on Sep. 9, 2003 entitled DIRECLTY COOLED THERMAL BARRIER COATING SYSTEM both show turbine airfoil with TBC applied thereto that make use of cooling channels formed under or partial with the TBC layer to provide extra cooling for the TBC. However, none of these two patents provide any reinforcement that will allow for a thicker layer of the TBC.
U.S. Pat. No. 4,629,397 issued to Schweitzer on Dec. 16, 1986 entitled STRUCTURAL COMPONENT FOR USE UNDER HIGH THERMAL LOAD CONDITIONS shows a turbine airfoil with unobstructed cooling ducts arranged along the airfoil in the spanwise direction with an air permeable metal felt layer and an air impermeable ceramic layer formed on top of the ducts. This method provides cooling to the TBC, but does not allow for a thicker layer of TBC. The above cited prior art references show airfoils with TBC layers that are cooled by air, but do not allow for a thicker layer that will result in spallation.
It is therefore an object of the present invention to allow for a thicker layer of the TBC to be applied that will not chip or break away (spallation) due to the thermal stresses developed in the layer.
It is another object of the present invention to provide for cooling of the TBC such that if a piece of the TBC was to break away (spallation) the remaining exposed part will be cooled by passing cooling air through the resulting hole to limit infusion of the hot gas flow.
It is another object of the present invention to provide for a turbine airfoil with a thicker TBC and a cooling hole that will provide cooling to the airfoil if a piece of the TBC was to break away.
It is another object of the present invention to provide for a method of making a turbine airfoil with a reinforced TBC applied thereto the airfoil.